In the potentiometric measurement of the partial pressure of a gas which in an aqueous solution generates an acid or a base, an electrochemical measuring electrode device is employed which, in accordance with the Stow-Severinghaus principle, comprises a potentiometric electrode system including a pH-electrode and a reference electrode, an electrolyte solution which communicates with the electrode system, and a membrane which is adapted to enclose the electrolyte solution in contact with the electrode system and which is permeable to the gas to be measured.
Correspondingly, in the polarographic measurement of the partial pressure of a gas, an electrochemical measuring electrode device is employed which, in accordance with the Clark-measuring principle, comprises a polarographic electrode system including a cathode and an anode, an electrolyte solution which communicates with the electrode system, and a membrane which is adapted to enclose the electrolyte solution in contact with the electrode system and which is permeable to the gas to be measured.
In operation of a Stow-Severinghaus electrode device for potentiometrically measuring the partial pressure of gas which in an aqueous solution generates an acid or a base, especially carbon dioxide, the gas in question permeates the membrane and is dissolved in the electrolyte solution, thereby causing a shift of pH, e.g.: ##STR1##
Correspondingly, in operation of a Clark-electrode device, e.g. when measuring the partial of O.sub.2, the gas to be measured permeates the membrane and is reduced at the cathode, i.e. the gas in question is consumed by the polarographic electrode system, e.g.: ##STR2##
Recent developments in electrochemical measuring electrode devices, especially for clinical measurement of gas partial pressures in blood, have provided devices for simultaneously measuring the partial pressures of two gasses, normally O.sub.2 and CO.sub.2, one of which is measured potentiometrically and another one of which is measured polarographically.
Thus, e.g. UK patent application No. 2.005.418 A discloses an electrode device for simultaneous measurement of pCO.sub.2 and pO.sub.2, in particular transcutaneously, and having a first electrode which is a pH-responsive electrode, a second electrode capable of electrochemically reducing oxygen, a reference electrode for each of, or common to, the pH-responsive electrode and the oxygen reducing electrode, an electrolyte in contact with the electrodes, and a membrane permeable to oxygen and carbon dioxide.
International Patent Application No. PCT/DK/81/00035 discloses an electrode device for measuring the partial pressure of oxygen and of a gas which in an aqueous solution generates an acid or a base, such as carbon dioxide, in particular transcutaneously, the device comprising potentiometric and polarographic measuring electrode systems of the above types and further comprising a compensation electrode adapted to electrochemically consume hydroxyl ions in an amount stoichiometrically equal to the amount of hydroxyl ions generated at the oxygen reducing electrode, i.e. the cathode in order to eliminate the influence of the hydroxyl ions generated at the oxygen reducing electrode on the pH of the electrolyte solution and, thus, on the measurement of the partial pressure of said gas which in an aqueous solution generates an acid or a base. In an article: "A Combined Transcutaneous pO.sub.2 -pCO.sub.2 Electrode with Electrochemical HCO.sub.3.sup.- Stabilisation", by John W. Severinghaus, published in Journal of Applied Physiology, vol 51, No. 4, pp 1027-1032, March 1981, a similar combined electrochemical measuring electrode device comprising a compensation electrode is disclosed.
The potentiometric measuring principle and the polarographic measuring principle are inherently different from each other. According to the potentiometric pH measuring principle, an equilibrium change caused by a change of the partial pressure of the gas in question is detected by measuring the change of pH in the electrolyte solution. Contrary to this, the polarographic measuring principle involves continuous consumption of the gas the partial pressure of which is to be measured; consequently, when measuring the partial pressure of a gas in a medium of limited gas availability, such as transcutaneous measurements, or measurements on small blood samples, problems may arise in correlating the current generated by reduction of the gas in question at the cathode of the polarographic electrode system and the steady state gas partial pressure outside the membrane since a large consumption of gas, which corresponds to a large current, influences the steady state outside the membrane and, thus, causes a decrease of the gas partial pressure outside the membrane, thereby incurring erroneous measuring results. For this reason, the membrane of polarographic electrode devices is normally constructed of a material showing relatively low permeability to the gas to be measured in order to reduce the consumption of the gas in question.
The present invention provides an electrochemical measuring electrode device which permits highly accurate and highly responsive measurements of the partial pressures of two gasses in a medium of limited gas availability one of which is measured potentiometrically and another one of which is measured polarographically.